1. Field of the Invention
This invention relates to a process for the production of methyl ethers. More particularly this invention relates to such process in which methanol is reacted with a branched olefin in the presence of an acid catalyst and an additive which prevents or inhibits polymerization of the olefin.
2. Prior Art
Methyl ethers derived from branched monoolefins such as methyl-t-butyl ether are known additives to gasoline which improve the octane rating of mixtures. Such methyl ethers are produced by reacting methanol with isobutylene, or the corresponding higher branched monoolefin, in the presence of an acidic catalyst. The reaction can proceed with mixed hydrocarbon streams containing the branched monoolefin, with most catalytic systems being generally economically limited to hydrocarbon streams containing at least about 50% of the desired branched monoolefin, typically in combination with other monoolefins, diolefins and alkanes.
Among the common catalysts for the reaction are zeolites, acidic ion exchange resins and acidic inorganic materials. The reaction is normally conducted either in a batch fashion or with co-current feed of methanol and hydrocarbon. Unfortunately, the most common source of branched monoolefins, such as isobutylene, is from refinery operations wherein the isobutylene is present as less than 50% of a hydrocarbon stream, which often contain butadiene. While linear monoolefins and alkanes (e.g., butane) are inert in the presence of methanol and such acidic catalysts, the presence of butadiene complicates the reaction with most conventional catalysts, producing by-products such as polybutadienes.
U.S. Pat. No. 4,219,569 discloses a process of providing methyl-t-butyl ether (MTBE) suitable for gasoline-blending and C-4 hydrocarbons depleted of isobutylene suitable for alkylation. This patent indicates that simple distillation will not produce a C-4 overhead free of methanol in amounts objectionable for alkylation. Therefore, the patent recommends passing the distillate through a solvent such as ethylene glycol to lower its methanol content to below 100 ppm. This process suffers from a number of disadvantages. For example, the process is complex in that it requires a three step distillation and stripping the thus causing capital expense. Moreover, a fourth step (methanol stripping of the MTBE) is required, if a methanol content of less than about 5 weight % is required.
U.S. Pat. No. 2,720,547 describes a process in which mixed butenes are fed adjacent to the bottom of a reaction column operating at -50.degree. C. to +50.degree. C. and a mixture of methanol and alkanesulfonic acids are fed adjacent the top of the column. An overhead stream (containing unreacted butenes and a bottoms stream containing alkanesulfonic acid catalyst, methyl-t-butyl ether product and small amounts of methanol to which excess methanol is added) are both fed from the reaction column to a fractionating column operating at lower pressure. Four streams are removed from the fractionating column, one of which is an azeotrope of methyl-t-butyl ether and methanol. Methanol is scrubbed from the methyl-t-butyl ether with water.
U.S. Pat. No. 5,454,356 discloses a process for producing methyl ethers of branched monoolefins. In this process, methanol is reacted with branched monoolefins, such as isobutylene, or mixtures of hydrocarbons such as mixed C-4's from a refinery, in the presence of methanesulfonic acid. In this process the reactor also functions as a fractionator.